The present invention relates to an X-ray generator that generates the X-ray and extreme ultraviolet (“EUV”) light, and an exposure apparatus having the same.
In manufacturing such a fine semiconductor device as a semiconductor memory and a logic circuit in the photolithography technology, a reduction projection exposure apparatus has been conventionally employed which uses a projection optical system that projects a circuit pattern formed on a mask onto a wafer, etc. to transfer the circuit pattern.
The minimum critical dimension (“CD”) to be transferred by the projection exposure apparatus or resolution is proportionate to a wavelength of light used for exposure. Thus, a projection optical apparatus using the EUV light with a wavelength of about 10 nm to about 15 nm much shorter than that of the UV light (referred to as “EUV exposure apparatus” hereinafter) has been developed. The EUV exposure apparatus uses a discharge type plasma light source that generates the plasma and generates the EUV light by introducing gas to the electrode for discharging. Such an EUV exposure apparatus is disclosed, for example, in Japanese Patent Publication, Application No. 2004-226244.
The EUV light source is used in a high vacuum similar to the mirror and mask. For example, the discharge type plasma light source generates the plasma by applying the high voltage to the electrode, and thus the electrode portion becomes at the high temperature. Although the cooling water chills the electrode, but the energy beyond the cooling capacity of the cooling water may need to be projected to continuously generate the high-intensity EUV light. However, the vacuum has no air around the electrode to radiate the heat, and the temperature of the electrode gradually rises and the continuous driving melts the electrode portion.
Measures that attempt to always maintain the light source in the normal temperature range and prevent damages of the electrode include a continuous emission method that decreases the applied voltage to the electrode and thus the EUV light intensity, and a method that introduces a downtime period and lowers the electrode temperature. However, both methods cause a drop of the throughput of the exposure apparatus. In order to expose without lowering the throughput, one disclosed method switches plural EUV light sources and cools the light source that is not being used for exposure. See, for example, Japanese Patent Publication, Application No. 2003-282424, which arranges four light sources 11 to 14 at intervals of 90°, as shown in FIGS. 9A and 9B, rotates a mirror 21, and introduces the EUV light to the subsequent illumination optical system.
The EUV exposure apparatus includes many mirrors, and each mirror's reflectance to the s-polarized light is higher than p-polarized light by several times. Since the p-polarized light is absorbed in the mirror and causes a generation of heat, effective use of the s-polarized light component of the incident light is vital to improve the use efficiency of the light. However, the prior art does not weigh the optimal polarization condition in switching the plural lights.
In other words, the prior art has a problem of fluctuation of the polarization plane, a surface on which the electric field vector oscillates, whenever the light source is switched. The fluctuation of the polarization plane becomes conspicuous when the light intensity is different between the s-polarized light and the p-polarized light reflected by the mirror 21. When the mirror 21 has a multilayer coating, the incident angle upon the mirror 21 (abscissa axis) and a ratio between the p-polarized light and the s-polarized light reflected by the mirror 21 (Rp/Rs) (ordinate axis) shows a characteristic shown in FIG. 2. It is understood from FIG. 2 that the p-polarized light component becomes 0 around 45°.
Since Japanese Patent Publication, Application No. 2003-282424 sets the incident angle upon the mirror 21 to about 45°, the reflected light on the mirror 21 becomes a linearly polarized light in which the electric field vector directs in the perpendicular direction to the optical axis of the light emitted from each light source. Therefore, whenever the light source is switched, the polarization plane rotates. For example, assume two planes shown in FIGS. 9A and 9B in Japanese Patent Publication, Application No. 2003-282424. The plane shown in FIG. 9A is a plane determined by the optical axis of the EUV light emitted form the mirror 21 and a line that connects the light source 11 to the mirror 21. The plane shown in FIG. 9B is a plane determined by the optical axis of the EUV light emitted form the mirror 21 and a line that connects the light source 12 to the mirror 21. When the mirror 21 switches the light source from 11 to 12 for, the plane rotates by 90° and the s-polarized light perpendicular to the plane rotates by 90°.
If the light emission state shown in FIG. 9A is set so as to provide the subsequent illumination optical system with the highest s-polarized light state, the emission state shown in FIG. 9B rotates by 90° and the use efficiency of the illumination optical system exhibits the lowest s-polarized light. As a result, the throughput drops in the emission state shown in FIG. 9B. When the mirror 21 is rotated at a predetermined pulse, the mask is illuminated with an oscillation of strong and weak intensities and thus the exposure dose fluctuates. As a result, some of plural patterns having the same CD are exposed, but other patterns are not.